Geometry & Shapes (id: f9b7214ee)

本题目来源于试卷: Geometry & Shapes，类别为 IB数学

[问答题]
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A farmer wants to create an enclosure for his chickens, so he has purchased 28 meters of chicken coop wire mesh.
1. Initially the farmer considers making a rectangular enclosure.
1. Complete the following table to show all the possible rectangular enclosures with sides of at least $4 \mathrm{~m} $ he can make with the 28$ \mathrm{~m} $ of mesh. The sides of the enclosure are

2. What is the name of the shape that gives the maximum area?

The farmer wonders what the area will be if instead of a rectangular enclosure he uses an equilateral triangular enclosure.
2. Show that the area of the triangular enclosure will be $\frac{196 \sqrt{3}}{9}$ .

Next, the farmer considers what the area will be if the enclosure has the form of a regular pentagon. The following diagram shows a regular pentagon.

Let O be the centre of the regular pentagon. The pentagon is divided into five congruent isosceles triangles and angle $\mathrm{A} \widehat{O}$ B is equal to $ \theta $ radians.
3. 1. Express $\theta$ in terms of $\pi$ .
2. Show that the length of $\mathrm{OA}$ is $\frac{14}{5} \operatorname{cosec}\left(\frac{\pi}{5}\right) \mathrm{m}$.
3. Show that the area of the regular pentagon is $\frac{196}{5} \cot \left(\frac{\pi}{5}\right) \mathrm{m}^{2} $.

Now, the farmer considers the case of a regular hexagon.
4. Using the method in part (c), show that the area of the regular hexagon is

$\frac{196}{6} \cot \left(\frac{\pi}{6}\right) \mathrm{m}^{2}$


The farmer notices that the hexagonal enclosure has a larger area than the pentagonal enclosure. He considers now the general case of an n -sided regular polygon. Let $ A_{n} $ be the area of the n -sided regular polygon with perimeter of 28 $\mathrm{~m} $.
5. Show that $A_{n}=\frac{196}{n} \cot \left(\frac{\pi}{n}\right)$ .
6. Hence, find the area of an enclosure that is a regular 14 -sided polygon with a perimeter of 28 $\mathrm{~m}$ . Give your answer correct to one decimal place.
7. 1. Evaluate $\lim _{n \rightarrow \infty} A_{n}$ .
2. Interpret the meaning of the result of part $(\mathrm{g}) (i)$.

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